This invention relates to a rotary control damper adapted to move in a direction in which a rotary body such as a cassette holder where a cassette tape for a cassette tape recorder is contained or a compact disk holder where a compact disc for a compact disc player is contained is pivotally opened or closed while a rotary energy of the rotary body when it is opened or closed about a support pivot is absorbed or damped by viscosity resistance of an oil such as silicone grease with which the damper is filled.
What is illustrated in FIGS. 11 and 12 is a cassette holder into which the rotary control damper is incorporated. A cassette holder 2 is provided in a body of a cassette tape recorder 1 and serves to contain a cassette tape so that it can be taken out of the cassette holder 2. The cassette holder 2 is pivotally moved about a support shaft 3 relative to the body of the cassette tape recorder 1 so that it is opened or closed in a forward or backward direction.
A return spring 4 is provided about the support shaft 3 so that the cassette holder 2 is normally urged in the forward direction. A sector gear 5 is securely provided on a side face of the cassette holder 2. A rotary control damper 6 is securely provided on the body of the cassette tape recorder 1 and can rotate while it threadedly engages the sector gear 5.
The rotary control damper 6 has been disclosed in Japanese Utility Model Application Publication No. 37,236/1989 and has such a construction as shown in FIG. 13. The rotary control damper 6 comprises a driven gear 7 having two cylindrical protrusions 8 coaxially provided on a bottom of the driven gear 7. A bottomed case 9 has a cylindrical wall 10 in which the two cylindrical protrusions 8 of the driven gear 7 are placed. A space or gap exists between the outer cylindrical protrusion 8 and the cylindrical wall 10 and between the inner cylindrical protrusion 8 and an annular wall 10A provided inside of the cylindrical wall 10 on the bottomed case 9 that is filled with an oil 11. An annular shoulder 12 is formed on the inner periphery of the cylindrical wall 10 at the middle portion thereof so that an inner diameter of the upper portion of the annular shoulder 12 is larger than an inner diameter of the lower portion of the annular shoulder 12. Excess oil 11 placed in the space between the two cylindrical protrusions 8 and the walls 10 and 10A is stored in the upper portion of the annular shoulder 12.
When the cassette holder 2 is unlocked, it is pivotally opened about the support shaft 3 by the stability of the return spring 4 in the forward direction from the position indicated by the solid line of FIG. 11 to the position indicated by the dotted line thereof. Thus, the driven gear 7 of the rotary control damper 6 rotates because it engages the sector gear 5 moving together with the cassette holder 2. At that time, the rotary energy of the cassette holder 2 is absorbed through the driven gear 7 by the viscosity resistance of the oil 11 with which the gap between the cylindrical protrusions 8 and the cylindrical wall 10 or annular wall 10A is filled. Accordingly, the cassette holder 2 slowly opens.
In order to accomplish the aforementioned function of the rotary control damper 6, the gap between the cylindrical protrusions 8 of the driven gear 7 and the cylindrical wall 10 or the annular wall 10A of the bottomed case 9 should be desirably uniform all over the periphery thereof. However, as an external force is applied in a horizontal direction (as viewed in FIG. 13 )to the driven gear 7 engaging the sector gear 5 of the rotary control damper 6 which moves together with the cassette holder 2 when it is opened, the cylindrical protrusions 8 move in a direction identical to the direction in which the external force is applied.
Although the oil 11 with which the gap between the cylindrical protrusions 8 of the driven gear 7 and the cylindrical wall 10 or the annular wall 10A of the bottomed case 9 makes such resistance as prevents the cylindrical protrusions 8 from moving, it is impossible to maintain the gap between the cylindrical protrusions 8 and the cylindrical wall 10 or the annular wall 10A uniform all over the periphery thereof. Thus, the cylindrical protrusions 8 of the driven gear 7 move in the direction identical to the direction in which the external force is applied. Therefore, a center of the cylindrical protrusions 8 of the driven gear 7 never accords with the center of the cylindrical wall 10 or the annular wall 10A of the bottomed case 9.
As a result, the driven gear 7 is deflected as it rotates and it cannot be pivotally operated in a smooth manner. Furthermore, since the size of the gap between the cylindrical protrusions 8 of the driven gear 7 and the cylindrical wall 10 or the annular wall 10A of the bottomed case 9 is uneven along its periphery, the thickness of the oil 11 with which the gap is filled is not uniform, which disadvantagesouly causes the rotary torque to be lowered.
In addition, since the oil 11 in that portion of the gap made narrower as the driven gear 7 rotates is compressed, there occurs a reaction force of the oil 11 which causes the wall face defining the gap which is filled with the oil 11 to be forced in an upward or a downward and a leftward or a rightward direction. Therefore, the driven gear 7 tends to be urged in a direction far away from the stationary bottomed case 9 (in an upward direction in FIG. 13). This possibly causes the cylindrical protrusions 8 of the driven gear 7 to be removed out of the cylindrical wall 10 or the annular wall 10A of the bottomed case 9 and therefore prevents the rotary control damper 6 from being pivotally operated in a smooth manner.